This invention relates to FM broadcasting systems and, more particularly, to transmission techniques for increasing the channel capacity and coverage area of FM stereophonic broadcasting systems for both radio and television applications while maintaining compatability with existing monophonic and stereophonic radio receivers and with the transmission standards for stereo television recently adopted by the Electronic Industries Association (EIA).
The potential of FM sound broadcasting has long been recognized, primarily due to its relative immunity to electromagnetic interference and its ability to provide full audio bandwidth with low noise. Although FM stereo adds a new acoustical dimension to radio reception, it does so only at the expense of serious degradation of signal-to-noise ratio. The noise penalty in stereophonic broadcasting is well known, there being several factors which contribute to the higher noise levels and coverage losses resulting from multi-channel sound transmissions. When a broadcast station converts to biphonic service, monophonic coverage is reduced because signal power must be divided among the various components of the more complex baseband signal. (The term "biphonic" will be used hereinafter to clearly differentiate two-channel broadcasting from other forms of stereophony such as triphonic and quadraphonic broadcasting.) The biphonic signal-to-noise ratio is lower than monophonic signal-to-noise ratio because of the wide bandwith of the composite signal containing the monophonic sum signal M, the pilot signal p, and the stereophonic difference signal S. With a baseband spectrum extending to 53 kHz for biphonic transmissions, the noise level is particularly high because of the rising spectral characteristic due to frequency modulation. The so-called "triangular" noise spectrum increases 6 dB per octave with increasing frequency of the composite signal, and although audio de-emphasis counteracts this somewhat, the noise problem is still severe. After demodulation, the noise components of the difference channel sub-carrier are added, statistically independent, to the noise already present in the monophonic signal during audio dematrixing.
Instead of here describing the factors that must be taken into account in estimating the theoretical loss of signal-to-noise ratio, attention is directed to U.S. Pat. No. 4,485,483 dated Nov. 27, 1984 of Emil L. Torick and Thomas B. Keller, the disclosure of which is hereby incorporated herein by reference, for a summary of a number of studies that have been made of signal-to noise degradation. In general, these studies conclude that there is a 26 dB penalty for stereophonic programming with wide audio separation, whereas for monophonic receivers noise degradation is in the range from 1 dB to 7 dB. Such losses of signal-to-noise ratio result in a reduction in the effective area of coverage of a broadcast station, studies having shown that with biphonic transmission the two-channel reception range is only approximately one-half the monophonic reception range.
The above-mentioned patent describes a biphonic FM radio broadcasting system that is fully compatible with existing receivers and which through improvement of signal-to-noise significantly extends the area of coverage of an FM broadcasting station. In the disclosed system (hereinafter sometimes referred to as the "Torick/Keller system") the usual left and right signals are conventionally matrixed to obtain conventional sum (M) and difference (S) signals. The difference signal is used to amplitude-modulate a first sub-carrier signal and at the same time is applied to a compressor which compresses its dynamic range to produce a compressed difference signal S'. The compressed difference signal S' is used to amplitude-modulate a second sub-carrier signal of the same frequency but in quadrature phase relationship with the first. Suppressed-carrier, double-sideband modulation of each sub-carrier is employed, with the frequency of the sub-carrier signal being sufficiently high to assure a frequency gap between the lower sidebands of the modulated sub-carrier signals and the M signal. A conventional low-level phase reference pilot signal, lying within the aforementioned frequency gap, is employed for detection purposes at the receiver. The M signal, the two modulated sub-carrier signals, and the pilot signal are frequency modulated onto a high frequency carrier for transmission purposes. The receiver includes a demodulator for deriving the M signal, the normal difference signal S and the compressed difference signal S', and an expander for complementarily expanding the derived compressed difference signal. The expanded noise-reduced version of the difference signal is combined with the derived sum signal M to obtain the original left (L) and right (R) signals. The receiver also includes switch means for applying the normal difference signal, instead of the expanded version of the derived difference signal, to the combining means to enable the receiver for reproduction of conventional stereophonic signals.
In effect, then, the Torick/Keller system embodies the concept of companding a channel that is additional to that normally used for stereo and to code it in a way so as to not increase the bandwidth requirements for transmission. By companding the difference (L-R) signal, which gives 22 dB to 26 dB signal-to-noise improvement in the transmission chain, the stereo listener theoretically enjoys the same signal-to-noise ratio as does the conventional monophonic listener. This amount of reduction of received noise greatly increases the effective stereo service area; in general, the radius from the transmitter to the point at which an acceptable signal is no longer received is at least doubled as compared to that for conventional stereo, which means that four times the number of potential listeners will receive an acceptable signal.
In the application of stereophonic sound to television according to the system recently adopted by the Electronic Industries Association (EIA), the difference signal (L-R) is compressed by a dbx Inc. compressor to give the signal-to-noise improvement necessary to overcome the penalty in signal-to-noise as compared to monophonic transmission. The compressed difference signal is used to amplitude-modulate a sub-carrier and the amplitude-modulated sub-carrier and the usual sum signal M, and a pilot signal, are frequency modulated onto a high frequency carrier for transmission purposes. Only the compressed difference signal is transmitted; that is, there is no transmission of an uncompressed difference signal. Thus, the signal can be compatibly received by existing monophonic television receivers, but cannot be received by existing FM stereo radio receivers; but since there had not previously been a standard for stereophonic television, compatibility was not a problem in the adoption of the EIA stereo television system.
A primary object of the present invention is to provide an FM stereophonic broadcasting system which exhibits a greater signal-to-noise improvement than that obtainable with the Torick/Keller system.
A corollary object is to provide an FM stereo broadcast system which better utilizes the greater channel capacity of the Torick/Keller system while still realizing its improved signal-to-noise advantage.
Still another object of the invention is to compatibly improve the stereo television system recently agreed to by the EIA, wherein only a compressed audio difference signal of reduced maximum amplitude is transmitted, by also transmitting an unchanged difference signal in quadrature.